Composites are widely used in the aerospace industry due to their excellent characteristics, such as high specific strength, specific stiffness, good fatigue resistance, and designability.
With the advancement of digital aircraft manufacturing technology and composite manufacturing processes, digital design and digital manufacturing are gradually becoming the core technologies of composite manufacturing.
Digital inspection of composite parts has become one of the key links to control the quality of composite parts, combined with the digital manufacturing technology of composite materials.
The traditional inspection method for composite parts involves making inspection samples, which is costly and inefficient, with low precision.
However, with the development of digital measurement technology, portable coordinate measurement systems, especially the emergence of laser trackers, provide the necessary technical means for the digital inspection of composite parts.
System components and measurement principle of laser tracker measurement
The laser tracker, also known as a mobile CMM, is a portable coordinate measuring system based on the spherical coordinate system. It offers several advantages such as high measurement accuracy, real-time measurement, fast and dynamic measurement, and easy mobility.
Using the laser tracker, you can measure the distance to the target point and the horizontal and vertical deflection angles.
The basic principle behind the laser tracker is placing a reflector on the target position, emitting a laser shot to the reflector, and then tracking the return beam with the tracking head. As the target moves, the tracking head adjusts the beam direction to remain focused on the target. The detection system then receives the return beam and uses it to measure the spatial position of the target.
To sum up, the laser tracker is used to determine the spatial coordinates of a target point by measuring the position of a reflector placed on the target point. It directly measures the three-dimensional coordinates of the spatial point, which are obtained in the laser tracker’s instrument coordinate system.
The instrument coordinate system is defined as follows: the center of the tracking head serves as the origin, the direction of the 0 reading on the degree dial is the X-axis, the normal upward direction of the degree dial plane is the Z-axis, and the right-handed coordinate system determines the Y-axis.
Figure 1 illustrates the instrument coordinate system established according to the above requirements.
Figure 1 Laser Tracker measurement schematic
When the reflector moves away from the reference position (whose distance from the center of the instrument is known) and moves in space, the laser tracker automatically tracks the reflector and records the interferometric distance value D and the angle values α and β on the vertical and horizontal dials simultaneously.
Using these three observations, you can obtain the spatial 3D Cartesian coordinates (x, y, z) of the point according to equation (1).
Inspection of composite parts
An aircraft composite reinforcing rib was traditionally manufactured using analog techniques. However, with the increasing need for digital manufacturing, a laser tracker is required to detect any deviations from the theoretical position of this part.
In this article, a Leica AT901-LR laser tracker measurement system was used, with a measuring radius of 80m and a spatial length measurement uncertainty of 15μm + 6μm/m. The T-probe measuring radius was 15m, and the spatial length measurement uncertainty was 7μm/m.
The measurement process for the composite parts involved three steps: establishing the measurement modulus, establishing the measurement coordinate system, and conducting the measurement and analysis of the results.
1. Establishment of the measurement modules
Since this composite part is manufactured using traditional analog technology without a digital model, the first step is to create a measurement model of the part before digital measurements can be made.
The process of manufacturing large composite parts is different from that of traditional metal parts. Due to the characteristics of composite materials, a large number of high-precision machining cannot be performed at a later stage. Therefore, the molding of composite parts is characterized by integrated molding, and later precision machining is less frequent. As a result, the accuracy of the molding die plays a crucial role in ensuring the accuracy of the composite part.
The accuracy of the forming die for the composite parts discussed in this paper has been verified through inspection samples. Therefore, the forming die can be used as the basis for testing the composite parts.
To establish the measurement modules, the laser tracker is used to measure the composite molding mold, and the measurement results are used to create the measurement model. The process of establishing the measurement model involves measuring points on the die using a laser tracker, forming a measurement point cloud, and fitting a large number of measurement point clouds to a calculation to generate a profile. This profile can be used as the part’s measurement model since it matches the part’s profile.
2. Establishment of the measurement coordinate system
The measuring die needs to be measured in the same coordinate system as the composite part, which requires establishing a measuring coordinate system.
One commonly used method is to set fixed measurement points around the perimeter of the die when creating the measurement digital model. These points can be measured using a laser tracker, and their spatial coordinate measurements recorded.
The measurement points serve as the reference points for the measurement coordinate system, and all subsequent measurements of the part are based on these points as the original reference.
3. Measurement of parts
After positioning the composite part on the die, the die is removed.
Next, a laser tracker is utilized to measure the reference point which serves to maintain the part in the same coordinate system as the measurement die.
After unifying the coordinates, the part undergoes inspection. The measurement software automatically compares the measured data with the theoretical data of the measurement module by utilizing a reflector to directly measure the part profile. This process not only measures the geometric position of the part but also comprehensively evaluates the overall position of the state in the current coordinate system.
4. Results analysis
Various factors affect the measurement results, including instrument accuracy, vibration, part placement, and other aspects.
Instrument accuracy is typically a fixed factor. The composite parts require an accuracy of ±1mm, which is much higher than the accuracy of the laser tracker. Thus, the impact of instrument accuracy is almost negligible.
Vibration can be controlled by choosing the station of the laser tracker.
Part placement, on the other hand, is a significant factor in measurement results. During the measurement process, the position of the laminating part is determined by the forming die. However, during the installation process, it is impossible for the parts to fit perfectly with the die. Consequently, the placement of the parts results in a significant error.
To minimize the influence of part placement on the measurement results, the solution is to optimize the numerical fit by using the part itself as a reference. This will help reduce the impact of part placement on the measurement results.
Table 1 Measurement results before and after fitting optimization
| Deviation before optimization /mm | Deviation after optimization /mm | |
| measuring point 1 | 1.368 | 0.447 |
| measuring point 2 | 1.308 | 0.434 |
| measuring point 3 | 1.266 | 0.442 |
| measuring point 4 | 1.165 | 0.391 |
| measuring point 5 | 1.089 | 0.373 |
| measuring point 6 | 0.906 | 0.251 |
| measuring point 7 | 1.029 | 0.424 |
| measuring point 8 | 0.905 | 0.401 |
| measuring point 9 | 0.910 | 0.453 |
| measuring point 10 | 0.839 | 0.419 |
| measuring point 11 | 0.786 | 0.402 |
| measuring point 12 | 0.820 | 0.480 |
| measuring point 13 | 0.626 | 0.342 |
| measuring point 14 | 0.668 | 0.448 |
| measuring point 15 | 0.680 | 0.499 |
| measuring point 16 | 0.610 | 0.482 |
According to Table 1, the pre-optimization measurement results significantly deviate from the theoretical values. However, these values do not represent the actual condition of the part.
By optimizing the fit, the deviation of the measurement results is significantly reduced, thereby eliminating errors caused by part placement. The optimized results can then accurately reflect the true condition of the part.
Conclusion
The use of digital design and manufacturing technology has become an inevitable trend in the development of composite forming die design and manufacturing, as well as in rapid digital inspection.
This technology not only overcomes the limitations of traditional analog quantity transfer, but also provides an effective means of quality control for composite manufacturing.
